The present invention relates to a centrifugal pump, which has a housing holding one or more impellers. The impellers may be of axial or semiaxial, closed or open design. An intake channel is arranged in front of a first impeller, and a plurality of grooves distributed around the circumference are provided in the wall face of the intake channel.
In centrifugal pumps which have a high specific velocity, a significant locally limited increase in the respective net positive suction head (NPSH) curve often occurs in the delivery range of 65-80% of the design volume flow. Meanwhile, depending on the pump design, the respective curve of the Q-H characteristic line may additionally have an instability which is referred to in general as a break or discontinuity in the characteristic line or as a saddle.
Such characteristic line shapes are due to the formation of the so-called partial load vortex, which occurs when the volume flow is reduced in the outside range of an impeller intake. A partial load vortex has a significant influence on the oncoming flow to the impeller under which the impeller is subjected to blocking of the meridional flow cross section and experiences a high velocity component in the direction of rotation of the impeller (spiral co-rotation).
U.S. Pat. No. 4,239,453 (=DE 25 58 840) describes an approach for avoiding the disadvantages of a partial load vortex, in which a diffusor is arranged in front of an impeller intake. Using this approach, the direction of action of a partial load vortex is reduced before it can reach the components situated in front of the impeller intake and can cause their destruction.
Other measures for influencing a partial load vortex are described in U.S. Pat. No. 6,290,458 (=EP 1,069,315), particularly in the description of the prior art. The measures “casing treatment, separator or active control” either require additional units in the machine periphery (active control), or reduce the efficiency even at the optimum point of the machine (casing treatment), or are associated with increased structural complexity (separator). This publication itself proposes the use of a plurality of grooves, which are generally referred to as J-grooves in accordance with the published article “An Improvement of Performance-Curve Instability in a Mixed-Flow Pump by J-Grooves,” May 29-Jun. 1, 2001, New Orleans, La., FEDSM 2001-18077, Proceedings of 2001 ASME Fluids Engineering Division Summer Meeting (FEDSM '01), because of their curved J shape.
J-grooves are shallow grooves but in another embodiment they may also have a spatial curvature and are provided in the pump housing in the direction of flow upstream from and above the impeller blades which are designed to be open at the impeller intake. The deciding factor for the functionality of the grooves is that they must partially cover the outside diameter of the impeller. In the area of the impeller cover, the impeller must be designed to be open to obtain a connection between a fluid zone provided with a higher pressure in the area of the open impeller blades and the beginnings of the J-grooves provided above that. As a result of this design measure, a fluid-carrying connection to the oncoming flow zone situated upstream is created via the J-grooves. Due to the J-grooves arranged in the main direction of flow, the open impeller wheel blades permanently deliver a partial stream of fluid already pumped upstream from the impeller back into the area of the oncoming flow to the impeller. These J-grooves have the disadvantage that their return flow is always active over the entire operating range of the pump. Consequently, the peak efficiency of a pump equipped with J-grooves declines.
Another disadvantage is the interaction between the free impeller blade tips and the opposing groove parts of the J-grooves fixedly positioned on the housing, which leads to increased noise and vibration phenomena. The passage on page 2 of the aforementioned literature citation in conjunction with FIG. 3 and the respective explanation thereof describes how to reduce these phenomena. To do so, the ends of the J-grooves arranged above the free blade tips are joined together by a peripheral ring groove. Above this ring groove to be provided additionally in the housing, there is an equalization of pressure between the end faces of the individual J-grooves. Furthermore, a high level of manufacturing complexity is required to provide such J-grooves, which are curved in space and extend in the manner of a discontinuity from the intake area into a conical housing wall face with a constant diameter. Thus, this method of influencing the partial load vortex is associated with some major disadvantages.